Electrostatic chucks are used for securing substrates, such as semiconductor wafers, in vacuum chambers of plasma processing systems. Generally, an electrostatic chuck includes a dielectric body and one or more electrodes embedded within the dielectric body. A chucking voltage applied to the electrodes establishes a clamping force by operation of Coulomb's law. The clamping force attracts the substrate toward a wafer-supporting surface of the dielectric body and holds the backside of the substrate in contact with the wafer-supporting surface. The exposed surface of the clamped substrate is subjected to a plasma process including, but not limited to, plasma cleaning, plasma etching, and plasma-enhanced deposition. After the plasma process is concluded, the clamping voltage is removed to release or dechuck the substrate.
The uniformity of the plasma processing of the substrate's exposed surface is a function of the uniformity of ion flux and ion energy, among other variables. The electrostatic chuck in inductively-coupled plasma (ICP) source systems may be biased independently with radio-frequency power so that the ion energy at the substrate can be varied without varying the ion flux at the substrate. The uniformity of the ion flux at the substrate is primarily determined by the plasma density distribution. Plasma density distributions in a plasma processing system equipped with an ICP source exhibit a prominent central peak near the azimuthal centerline of the chamber and decrease radially from the central peak toward the sidewall, which is typically cylindrical. FIG. 1 shows the radial dependence of the electron density and plasma temperature in a prior art ICP processing system. Therefore, the uniformity of the plasma processing is reduced by the radial dependence of the plasma density distribution, which is of particular concern for large diameter substrates.
Conventional techniques for reducing the central peak and, thereby, improving the uniformity of the plasma density distribution, include incorporating antennas having multiple coil configurations, introducing additional magnetic fields, and tailoring the dimensions and material of the substrate support pedestal. Although these techniques improve the uniformity of the plasma density distribution, the contribution of the chamber sidewall to the radial decrease in the plasma density distribution remains unaffected. Increasing the diameter of the sidewall lessens the radial non-uniformity but adds significant cost to the manufacture of the plasma processing system.
As mentioned above, plasma losses originating from the chamber sidewall contribute significantly to nonuniformity of the plasma density distribution. Attempts have been made to compensate sidewall effects by increasing the dimensions of the ICP source dielectric window, which must be robust and thus expensive, and by adding control units, power supplies, and cooling systems, which also adds hardware complexity. In plasma processing systems for large dimension substrates, such corrective measures increase material and consumables expenses, add complexity, and result in an increased cost of operation. As a result, the overall cost of the plasma processing system is significantly increased.
Therefore, there is a need for apparatus and methods for adjusting a parameter related to plasma conditions at the substrate-supporting surface of an electrostatic chuck.